Final answer:
Pleural fluid fills the slitlike pleural cavity between the two pleurae, providing lubrication and surface tension for the expansion and recoil of the lungs.
Step-by-step explanation:
The substance that fills the slitlike pleural cavity between the two pleurae, providing lubrication and surface tension that assists in the expansion and recoil of the lungs, is pleural fluid. Pleural fluid is secreted by mesothelial cells from both pleural layers and acts to lubricate their surfaces, reduce friction, and create surface tension. This surface tension helps maintain the position of the lungs against the thoracic wall and allows them to fill with air during ventilation.